A single call-out treats whatever's active at that particular moment, and it does that job reasonably well. Pests present at the time of treatment get dealt with, and the immediate issue that prompted the call generally disappears within a week or two.
What a one-off visit doesn't do is address the underlying conditions that allowed pests to establish themselves in the first place. Entry points remain unsealed, seasonal risk factors stay unmonitored, and there's no ongoing check to catch a new problem before it becomes visible to staff or customers again.